Anisotropic etch method

ABSTRACT

A method to anisotropically etch an oxide/silicide/poly sandwich structure on a silicon wafer substrate in situ, that is, using a single parallel plate plasma reactor chamber and a single inert cathode, with a variable gap between cathode and anode. This method has an oxide etch step and a silicide/poly etch step. The fully etched sandwich structure has a vertical profile at or near 90° from horizontal, with no bowing or notching.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of application Ser. No.09/571,523, filed May 16, 2000, pending, which is a continuation ofapplication Ser. No. 08/909,229, filed Aug. 11, 1997, now U.S. Pat. No.6,133,156, issued Oct. 17, 2000, which is a divisional of applicationSer. No. 08/603,573, filed Feb. 20, 1996, now U.S. Pat. No. 5,958,801issued Sep. 28, 1999, which is a continuation of application Ser. No.08/194,134, filed Feb. 8, 1994, abandoned, which is a divisional ofapplication Ser. No. 07/574,578, filed Aug. 27, 1990, now U.S. Pat. No.5,201,993, issued Apr. 13, 1993, which is a continuation of applicationSer. No. 382,403, filed Jul. 20, 1989, now U.S. Pat. No. 5,271,799,issued Dec. 21, 1993.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to etching methods used in thefabrication of integrated electronic circuits on a semiconductorsubstrate such as silicon, particularly a single-chamber/single-cathode(in situ) method of anisotropically plasma etching a sandwich structureof an oxide, tungsten silicide, and polycrystalline silicon, orequivalent structure.

[0004] An electronic circuit is chemically and physically integratedinto a substrate such as a silicon wafer by patterning regions in thesubstrate, and by patterning layers on the substrate. These regions andlayers can be conductive, for conductor and resistor fabrication, orinsulative, for insulator and capacitor fabrication. They can also be ofdiffering conductivity types, which is essential for transistor anddiode fabrication. Degrees of resistance, capacitance, and conductivityare controllable, as are the physical dimensions and locations of thepatterned regions and layers, making circuit integration possible.Fabrication can be quite complex and time consuming, and thereforeexpensive. It is thus a continuing quest of those in the semiconductorfabrication business to reduce fabrication times and costs of suchdevices in order to increase profits. Any simplified processing step orcombination of processes at a single step becomes a competitiveadvantage.

[0005] 2. State of the Art

[0006] A situation where a process simplification is desirable is in theanisotropic etch of a layer of oxide on a layer of silicide on a layerof poly (also called an oxide/silicide/poly sandwich structure). In thisdisclosure, “oxide” denotes an oxide of silicon and other commonly knownsilicides such as tungsten silicide, tantalum silicide, molybdenumsilicide, and titanium silicide, “silicide” is short for tungstensilicide, and “poly” is shoptalk for polycrystalline silicon. “Polycide”denotes a silicide-over-poly combination. Oxide is an insulator withdielectric properties. Poly is resistive in nature, but is made lessresistive when doped with an element having less or more than fourvalence electrons, or when layered with conductive silicide.

[0007] An oxide/silicide/poly sandwich structure presents a difficultetching task, particularly with an additional mask layer of photoresist(“resist”), which must be the case if patterning is desired. Thedifficulty is due to the distinct differences in the way oxide andpolycide are etched, particularly with resist still present on top ofthe structure.

[0008] Both oxide and polycide can be etched using a parallel plateplasma reactor. However, an oxide is typically etched in fluorinedeficient fluorocarbon based plasmas, whereas silicide and poly can beetched in fluorine or chlorine based discharges. Reactor cathodematerials may also differ: for oxide etch, an erodible cathode such asgraphite or silicon is often used to provide a source of carbon orsilicon for etch selectivity, whereas for polycide etch, an inertcathode is preferred, especially when utilizing chlorine gas (Cl₂) forselectivity. If a single-chamber process were attempted usingconventional art to etch an oxide/silicide/poly sandwich structure, theerodible cathode required for oxide etch would be destroyed by thechlorine required for polycide etch. Using conventional methods, the twosteps are incompatible.

[0009] Oxide etch in general is fairly well understood given a universalneed for a vertical profile. This vertical profile is realized primarilyby ion induced reaction with the oxide, coupled with normal incidence ofthe ions on the oxide surface. The amount and energy of these ions areprimarily controlled by the reactor's rf power and gap. Generally, afluorocarbon-based gas mixture is introduced at a low pressure into theetch chamber. The exact gas composition is chosen, and an erodiblecathode is used to scavenge excessive fluorine radicals so that thefluorine-to-carbon ratio is near, but not beyond, the so-calledpolymerization point. Under these conditions, when a plasma is ignited,the fluorocarbons are dissociated and release fluorine radicals andCF_(x) species. Although fluorine radicals etch oxide, they do so veryslowly: the primary etchant for oxide is considered to be the CF_(x)species. Some of these species diffuse to the oxide surface where, withthe assistance of ion bombardment, they react with the oxide and releasevolatile byproducts SiF₄, CO, and CO₂. In addition, some of the CF_(x)species react with each other to form fluorocarbon polymers. Polymerthat forms on horizontal surfaces is removed by vertical ionbombardment. Polymer that forms on vertical sidewalls is notsignificantly degraded by the bombardment, and actually serves a usefulpurpose by protecting the sidewalls from attack by the etchant species.This sidewall protection enables the achievement of vertical profiles,adjustable by varying the fluorine-to-carbon ratio. As the cathode iseroded, the quantity of available fluorine radicals is reduced.Therefore, a polymer-producing gas such as CHF₃ is balanced against afluorine-producing gas such as CF₄ to provide proper selectivity, withassistance to sidewall protection.

[0010] Two methods are presently used to etch an oxide/silicide/polysandwich structure. Both methods use separate reactors: one for oxideetch and one for polycide etch. The first method involves etching theoxide in an oxide etch reactor, using an erodible cathode. After oxideetch, the resist is removed from the wafer. Silicide and poly are thenetched in a poly etch reactor, using an inert cathode. Both etches areanisotropic.

[0011] The second method uses the same principles as the first, exceptthat there are two reactors in one machine. The two reactors areconfigured as separate oxide and polycide reactors having a commonvacuum transfer area, so that a wafer can be transferred in a vacuumfrom the oxide reactor to the polycide reactor, thus minimizingadditional handling. The resist is generally not removed prior topolycide etch in this method. This is sometimes referred to as “in situ”since the wafers never leave the vacuum of one machine. However they areetched in two different etch chambers and are not truly in situ in thesense of this disclosure.

[0012] It would be of great advantage to etch oxide and polycide truly“in situ,” that is, in one reactor chamber, with a single cathode.

BRIEF SUMMARY OF THE INVENTION

[0013] An object of the present invention is to provide a method ofanisotropically etching an oxide/silicide/poly sandwich structure insitu. Other objects of the invention are a fast processing time, animproved process yield and cleanliness.

[0014] In summary, the inventive process is as follows. A wafer havingthe sandwich structure described above, coated with a mask layer ofresist, is transferred into the chamber of a parallel plate plasmareactor, having an anodized aluminum cathode and a variable gap, for twosteps: oxide etch and polycide etch. In the oxide etch step, oxide notprotected by resist is exposed to a plasma of about 1.9 W/cm² powerdensity at a 0.48 cm gap, in a 2.3 torr atmosphere of 50 sccm C₂F₆, 100sccm He, 40 sccm CF₄, and 32 sccm CHF₃. Immediately after the oxide etchstep, in the same chamber and using the same cathode, silicide and polylayers are etched in a plasma of about 0.57 W/cm² at a 1.0 cm gap in a0.325 torr atmosphere of 90 sccm Cl₂ and 70 sccm He. The finishedstructure has a vertical profile at or near 90° from horizontal, with nobowing or notching. The entire inventive process takes about 3 minutes.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0015]FIG. 1 shows a cross-sectioned oxide/silicide/poly sandwichstructure with a patterned resist mask layer, prior to the inventiveetch.

[0016]FIG. 2 shows a cross-section of said structure after oxide etch.

[0017]FIG. 3 shows a cross-section of said structure after polycideetch.

DETAILED DESCRIPTION OF THE-INVENTION

[0018] As illustrated in FIG. 1, a photoresist mask layer 10 is alignedand developed on a sandwich structure of oxide 11, silicide 12, and poly13 on gate oxide 14 of a silicon wafer substrate 15. Fabrication anddamasking of this structure are done by methods well known to thoseskilled in semiconductor design and processing, and hence are not fullydisclosed herein. The preferred embodiment of the inventive method iswell suited to etch a 3,000 angstrom layer 11 of TEOS oxide (an oxide ofsilicon, derived from tetraethylorthosilicate) on 1,200 angstroms oftungsten silicide 12 on 3,000 angstroms of poly 13.

[0019] The wafer having the masked structure is transferred into thechamber of a Lam 790 parallel plate plasma reactor, having an anodizedaluminum cathode, a variable gap, and a 13.56 MHz rf power plasmagenerator for an inventive process having two steps: oxide etch andpolycide etch. In the oxide etch step, oxide 11 not protected byphotoresist mask layer 10 is exposed to a plasma of about 1.9 W/cm²power density at a 0.48 cm gap, in a 2.3 torr atmosphere of 50 sccmC₂F₆, 100 sccm He, 40 sccm CF₄, and 32 sccm CHF₃. In this disclosure,“sccm” denotes standard cubic centimeters per minute, and “gap” refersto the distance between plasma electrodes, one of which supports thewafer. After the oxide etch step, which takes under a minute, thestructure appears as shown in FIG. 2.

[0020] Immediately after the oxide etch step, in the same chamber andusing the same cathode, silicide and poly layers 12 and 13 are etched ina plasma of about 0.57 W/cm² at a 1.0 cm gap in a 0.325 torr atmosphereof 90 sccm Cl₂ and 70 sccm He. This etch takes a little over 2 minutes,with the entire inventive process taking about 3 minutes. The finishedstructure appears as shown in FIG. 3, with a profile at or near 90° fromhorizontal, with no bowing or notching.

[0021] Details of the oxide etch step are now provided. Althoughpreferred parameter values are stated above, plasma power density canrange within about 0.18-4.0 W/cm², the gap can vary within about 0.3-0.6cm, 0.38-0.52 cm being the preferred range, and the pressure can rangewithin about 1.8-3.0 torr, although 2.2-2.4 torr is preferred. Gasquantities may vary, as long as at least about 5 sccm He is provided.Providing more CF₄ than CHF₃ makes a cleaner process, but this ratio canbe varied if desired.

[0022] The inventive process uses a non-erodible anodized aluminumcathode, which increases the amount of available fluorine radicals.According to conventional thought, in order to maintain the sameoxide-to-polycide selectivity as the prior art, the ratio of CF₄ to CHF₃must be decreased to minimize fluorine radicals. It was found that thisapproach does not provide adequate selectivity without an excessive andquick buildup of polymer. This was solved by adding C₂F₆ to the chamberatmosphere as the predominant gas, which provides more CF_(x) speciesand relatively few fluorine radicals, resulting in acceptableselectivity without excessive polymer buildup. C₂F₆ also resolves a“micromasking” problem, in which areas of underlying polycide were notbeing etched. Although the cause is unclear, it is speculated that theCF_(x) species reacted with the tungsten silicide, forming a polymerlayer which interfered with subsequent polycide etching. C₂F₆ evidentlyproduces a polymer without this affinity for tungsten silicide, therebyeliminating micromasking.

[0023] The inventive process includes a high pressure atmosphere inorder to produce a faster oxide etch rate. High pressure results in ahigher fluorine radical flux on the oxide surface. When combined withhigh rf power, the etch rate is increased. High pressure and rf power dohave drawbacks, however. Although rf induced ion bombardment assists inoxide etch, it also contributes to photoresist erosion, which isundesirable. Further, if rf power is too high, the resist will “burn” orreticulate. Higher pressure makes a thicker atmosphere, scattering ionsand gas radicals in the plasma, resulting in more sidewall etching thanwith a low pressure system.

[0024] The oxide etch step of the inventive method includes an overetchof about 45 seconds to fully clear all residual oxide. Although theC₂F₆/CF₄/CHF₃ gas mixture etches underlying polycide during overetch,the etch continues to be anisotropic because of the sidewall passivationprovided by the halocarbon-derived polymer and from the carbonintroduced by eroding resist. After oxide has been cleared, thepolycide-to-resist etch rate ratio is approximately 1.8:1.

[0025] Polycide etch step details are now provided. Although preferredparameter values have been stated, plasma power density can range withinabout 0.18-2.0 W/cm², the gap can vary within about 0.5-2.5 cm, 0.8-1.5cm being the preferred range, and the pressure can range within about0.200-0.550 torr, although about 0.300-0.425 torr is preferred.Quantities of the gases may vary, as long as at least about 50 sccm Heis provided. It is contemplated that SiCl₄ or BCl₃ or a combinationthereof might be used to provide additional Cl₂, if desired.

[0026] The lower pressure of the polycide etch allows for more ionbombardment, which, with resist erosion and the Cl₂ concentration,determines the etch rate and profile of tile silicide and poly layers 12and 13. Cl₂ provides the necessary selectivity to the polycide, so thatminimal underlying gate oxide 14 is etched. Fluorine can also be used,but Cl₂ is preferred because it provides superior selectivity. Theresist used must therefore be able to reasonably withstand a chlorinebased plasma. The preferred embodiment utilizes Hunt's 6512 resist,developed with Hunt's photoresist developer 428. It is realized thatother resists, developers, and mask layer compositions can be used aswell.

[0027] An additional benefit of the inventive method is the ability touse carbon generated by the resist to help passivate polycide sidewalls,which means that carbon-containing gases do not have to be added to thegas mixture during polycide etch.

[0028] There is an upper rf power limit that can be safely used beforethe poly-to-gate oxide selectivity is reduced to the point where thepoly cannot be completely etched without removing all of the exposedgate oxide. The inventive process provides a selectivity ofapproximately 13:1. Variations in the chlorine flow and total pressuredo not significantly change this selectivity, although an increase in rfpower reduces it.

[0029] In both of the inventive steps, helium is added to improve etchuniformity. The pressure, power, and various gas quantities are balancedto produce the fastest etch rates while preserving selectivity.

[0030] Clearly, in view of the above disclosure, other embodiments ofthis invention will present themselves to those of ordinary skill insemiconductor processing, such as applying the invention to other kindsof masking layers, oxide, silicide, commonly referred to as tungstensilicide, tantalum silicide, molybdenum silicide, and titanium silicide,and poly, and varying thickness and doping of each layer etched. Sincethe inventive process includes one step for polycide etch, a simpleoxide/poly structure can be etched instead of an oxide/silicide/polystructure, without materially altering the process. It is alsoconceivable that plasma power density and gap may be varied, gasquantities adjusted, similar gases substituted, or some other inertmaterial used for the cathode, to achieve the same or similar results.Gas quantities may also be changed while preserving essentially similarratios of one gas to another. Another make of reactor might also bechosen. These variations and others are intended to be circumscribed bythese claims.

What is claimed is:
 1. A method to anisotropically etch at least onelayer in the group of layers consisting of silicide, polycrystallinesilicon, and polycide located over a layer of gate oxide on a substrate,said anisotropic etching producing a profile at or near 90° fromhorizontal in a parallel plate plasma etch reactor having a firstelectrode and having an inert second electrode comprised of anodizedaluminum, said method comprising: providing a gap of approximately 1.0cm between said first electrode and said inert second electrode;mounting said substrate on said first electrode; providing a plasmaatmosphere within said parallel plate plasma reactor, said plasmaatmosphere including a pressure of approximately 0.325 torr, includingCl₂ at a rate of approximately 90 sccm, He at a rate of approximately 70sccm, and a plasma power density of approximately 0.57 W/cm²; andanisotropically etching said at least one layer in the group of layersconsisting of silicide, polycrystalline silicon, and polycide locatedover said layer of gate oxide.
 2. A method to anisotropically etch atleast one layer in the group of layers consisting of a silicide layer, apolycrystalline silicon layer, and polycide layer located over a layerof gate oxide on a substrate, said anisotropical etch producing aprofile at or near 90° from horizontal with respect to said substrate ina parallel plate plasma etch reactor having a first electrode and aninert second electrode comprised of anodized aluminum; said firstelectrode and said inert second electrode having a gap therebetween,said method comprising: providing a gap within the range ofapproximately 0.5 cm to 2.5 cm; mounting said substrate on said firstelectrode; and providing a plasma atmosphere within said parallel plateplasma reactor including Cl₂ and He, a pressure within the range ofapproximately 0.200 to 0.550 torr, a plasma power density within therange of approximately 0.18 to 2.0 W/cm²; and anisotropically etchingsaid at least one layer in the group of layers comprising silicide,polycrystalline silicon, and polycide located over said layer of gateoxide.
 3. A method to anisotropically etch at least one layer in thegroup of layers consisting of a silicide layer, a polycrystallinesilicon layer, and a polycide layer located on a portion of a substrate,said anisotropical etch producing a profile at or near 90° fromhorizontal with respect to said substrate of said at least one layer ofsaid group of layers using a parallel plate plasma etch reactorincluding a first electrode and an inert second electrode comprises ofanodized aluminum, said first electrode and said inert second electrodehaving a gap therebetween, said method comprising: providing a gapwithin the range of approximately 0.8 cm to 1.5 cm; mounting saidsubstrate on said first electrode; and providing a plasma atmospherewithin said parallel plate plasma reactor including Cl₂ and He, apressure within the range of approximately 0.300 torr to 0.425 torr, anda plasma power density within the range of approximately 0.18 W/cm² to2.0 W/cm² to anisotropically etch said at least one layer in the groupof layers comprising a silicide layer, a polycrystalline silicon layer,and a polycide layer located on said portion of a substrate.
 4. A methodto anisotropically etch a structure in situ to produce a profile at ornear 90° from horizontal with respect to said structure, said structureincluding a first layer of an oxide of silicon on a second layerselected from the group consisting silicide, polycrystalline silicon,and polycide, said structure located on a substrate using a parallelplate plasma etch reactor including a first electrode and an inertsecond electrode comprises of anodized aluminum, said first electrodeand said inert second electrode having a gap therebetween, said methodcomprising: placing said substrate on said first electrode; providing afirst high pressure atmosphere within said parallel plate reactor, saidfirst high pressure atmosphere including C₂F₆, CHF₃, CF₄, and He;exposing the first layer to a first plasma having a first high powerdensity to expose at least a portion of said second layer; overetchingthe first layer to substantially remove the oxide of silicon; providinga second pressure atmosphere within said reactor including Cl₂ and He;and exposing the second layer to a second plasma having a second highpower density.
 5. The method of claim 4, wherein: said first highpressure atmosphere includes a pressure of approximately 2.3 torr, C₂F₆at the rate of approximately 50 sccm, CHF₃ at the rate of approximately32 sccm, CF₄ at the rate of approximately 40 sccm, and He at a rate ofapproximately 100 sccm; said first plasma including a power density ofapproximately 1.9 W/cm²; a gap of approximately 0.48 cm between saidfirst electrode and said inert second electrode for said first plasmapower density; said second high pressure atmosphere including a pressureof approximately 0.325 torr and Cl₂ at a rate of approximately 90 sccmand He at a rate of approximately 70 sccm; said second plasma includinga power density of approximately 0.57 W/cm²; and a gap of approximately1.0 cm between said first electrode and said inert second electrode forsaid second plasma power density.
 6. The method of claim 4, wherein:said first high pressure atmosphere including a pressure within therange of approximately 1.8 torr to 3.0 torr; said first plasma includinga power density within the range of approximately 0.18 to 4.0 W/cm²; agap within the range of approximately 0.3 to 0.6 cm between said firstelectrode and said inert second electrode for said first plasma powerdensity; said second high pressure atmosphere including a pressurewithin the range of approximately 0.200 torr to 0.550 torr; said secondplasma including a power density within the range of approximately 0.18to 2.0 W/cm²; and a gap within approximately 0.5 to 2.5 cm between saidfirst electrode and said inert second electrode for said second plasmahigh density.
 7. The method of claim 4, wherein: said first highpressure atmosphere including a pressure within the range ofapproximately 2.2 torr to 2.4 torr; said first plasma including a powerdensity within the range of approximately 0.18 to 4.0 W/cm²; a gapwithin approximately 0.38 to 0.52 cm between said first electrode andsaid inert second electrode for said first plasma power density; saidsecond high pressure atmosphere including a pressure within the range ofapproximately 0.300 torr to 0.425 torr; said second plasma including apower density within the range of approximately 0.18 to 2.0 W/cm²; and agap within approximately 0.8 to 1.5 cm between said first electrode andsaid inert second electrode for said second plasma power density.
 8. Themethod of claim 4, wherein: said first high pressure atmosphere includesmore C₂F₆ than CF₄ and more CF₄ than CHF₃; said first high pressureatmosphere includes at least approximately 5 sccm He; and said secondhigh pressure atmosphere includes He at the rate of at leastapproximately 50 sccm.
 9. The method of claim 4, wherein the structureincludes a mask layer that releases carbon when subjected to a plasmaand resists chlorine.